Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink is driven from the nozzle toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven by an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezoelectric element in the ink nozzle. Electrically-caused distortions of the piezoelectric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from the nozzle toward the print medium by the formation of an expanding vapor phase bubble in the nozzle. These various printing methods are described in “Output Hard Copy Devices,” edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13, entitled “Ink Jet Printing”).
The composition of the ink is traditionally comprised of deionized water, a water soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Several problems, however, are associated with soluble dyes that are not applicable to insoluble pigments. These problems include poor water-fastness, poor light-fastness, poor thermal stability, facile oxidation, dye crystallization, and ink bleeding and feathering on the print medium. To circumvent these problems, use of a pigment as the colorant is preferred. Pigments generally have better light-fast and water-fast properties, are more resistant to oxidation, and have higher thermal stability.
Pigment based inks have some disadvantages as compared to dye-based inks. Due to the insolubility of pigments in water, polymeric dispersants and/or surfactants are typically added to improve the dispersibility of the pigment. The addition of a polymeric dispersant can increase the viscosity of an ink and decrease its wetting ability. A viscous ink is more difficult to jet in a printhead.
Further, pigment based inks typically have increased dry time and reduced smear resistance as compared to dye based inks. Due to their size, pigment particles rest on the surface of the printed media as opposed to dyes, which penetrate into the media. As a result, pigment based inks require more time to dry before being handled to reduce image distortion.
To reduce smearing due to handling or highlighters, materials such as acrylic polymer resins have been added to pigment based inks. The acrylic polymer resins bind the pigment to the printed media. These resins can improve fixation of the pigmented inks to media, but they may simultaneously reduce the reliability and stability of the inks. For example, pigmented inks containing binder resins can cause the formation of ink material that blocks the vias and throats of a printhead.
Thus, there is a need for pigment based inks with improved smear resistance and improved reliability.